When people think of storm hazards they usually think of high winds and torrential rain, but in fact most deaths in coastal regions and even around large lakes are caused by the phenomenon known as a storm surge. If your area is threatened by a storm, it’s important to know what a storm surge is and how to stay safe from its effects.
A storm surge is a rise in sea level caused by the storm. There are two main ways in which this happens. Firstly, the low air pressure in the eye of a circular storm has an effect. The higher pressure outside the eye is pushing down on the water surface and this forces water into the eye, where a bulge forms in the surface. Sea levels can rise by two-fifths of an inch for every millibar (mbar) drop in air pressure, and it’s not uncommon for the eye of a storm to have air pressures around 960 mbar. As the average atmospheric pressure is 1013.25 mbar, this can result in sea levels rising by up to two feet. That doesn’t sound like much, but remember that this is the base sea level. A two foot rise can easily move the high tide mark fifty feet or more closer to houses, coast roads and other vulnerable structures. Add big storm-driven waves on top of the higher water and two feet can be enough for beachfront houses to suffer flooding and wave damage.
Worse is to come, though. The rise caused by low pressure is a fairly minor component of storm surge. A much bigger effect comes from the winds circling the eye, which literally pile up water in front of the storm. Tropical cyclones also tend to have rain bands surrounding them, and heavy rainfall over land can increase the surge further as rainwater flowing down rivers meets the incoming surge and backs up. Added together, the effects can be dramatic; the highest storm surge recorded during Hurricane Katrina was 27.8 feet. This is enough to overwhelm many sea barriers, even if it doesn’t break them, and the surge can reach several miles inland. Most coastal towns faced with a surge of much over five feet are going to experience widespread flooding and large areas will be exposed to violent waves; the US National Hurricane Center estimate that 72% of ports, 27% of major roads and 9% of railway lines in the USA’s Gulf coast region are at or below four feet above normal sea level.
A storm surge’s potential to destroy property is obvious, but it also causes many risks to life. Drowning is a self-explanatory one; surges cause sea levels to rise rapidly, which can quickly cover large areas in deep water. The water of a storm surge also tends to be in constant motion, which is a danger in itself. Even six inches of moving water can knock people over or sweep cars off the road, especially if it is heavily loaded with debris or mud. Once knocked off their feet, people can be swept into solid objects, struck by debris or drowned. The effect of water on vehicles causes many accidents.
Waves are also a danger. Anyone in the open can easily be knocked down by a wave, with the same hazards as listed above. Waves can also cause structural damage to buildings or even collapse them, which is a hazard to anyone trapped inside. Single-storey buildings can even be completely submerged by a storm surge.
Floodwater can damage electricity supplies, often causing fires. Other utilities such as water, gas and telephone systems are equally vulnerable. By transporting large quantities of seawater far inland a storm surge can also contaminate drinking water sources such as reservoirs. Longer-term salt damage to farmland is also an issue.
A further hazard comes from the effect of seawater flooding on wildlife. Animals such as alligators and snakes are often driven from their habitat by storm surge and can take refuge in homes or other areas where people are sheltering. These animals are likely to be agitated and potentially dangerous.
Obviously storm surge is a significant hazard to life. There are a number of precautions that should be taken by anyone in an area likely to be affected. Firstly, listen to weather reports and storm warnings. Keep a battery-powered radio in case mains power is knocked out by the storm. Keep an emergency kit containing a first-aid pack, containers of drinking water, high-energy food and battery-powered lanterns; if you have a multi-storey house this kit should be stored on the top floor, above any likely flooding. Make sure you know how to turn off the electricity and gas supply in case this is necessary. Keep essential medication and important documents handy in case you have to evacuate or take shelter upstairs. Most importantly, never ignore an evacuation order. If you are ordered to evacuate, there is a very high chance of your area being affected by a storm surge, and if you decide to stay put you are placing yourself in danger. It’s unlikely that anyone will be able to come and rescue you when the surge hits and at that point, it’s too late to leave; you won’t outrun the water.
